1. Field of the Invention
The present invention relates to a power converter (SMPS) that provides DC power required to operate general electronic devices such as computers, TVs and LED lamps, and more particularly, to a half-bridge power converter for driving an LED by series-resonant connection of an inductor, an inductor and a capacitor (referred to as LLC half-bridge power converter for driving an LED) that uses a CMOS timer IC having 50% duty cycle and a usable frequency range of 50 KHz through 1 MHz as a controller and varies the capacitance of a frequency variable condenser of the CMOS timer IC based on an output level to maintain a rated output.
2. Background of the Related Art
Current power converters for driving LED lamps do not meet high efficiency and low energy consumption because specifications of LEDs are unsettled and the power converters do not satisfy required conditions. This is caused by problems of LEDs and, particularly, short lifetime and high error rate of the power converters. Considering this circumstance, a problem calling for immediate solution is to secure high efficiency and high stability of power converters.
Flyback, forward and half-bridge power converters are typical PWM power converters currently most widely used. These power converters are easily designed when a frequency of lower than 100 KHz is applied thereto. In this case, however, efficiency of higher than 80% is difficult to achieve and the volume and manufacturing cost of the power converters increase.
An LLC (Inductor, Inductor and Capacitor) half-bridge power converter, which has been recently commercially used, is known as a power converter for driving LED lamps.
FIG. 1 is a circuit diagram of a conventional LLC half-bridge power converter for driving an LED.
Referring to FIG. 1, the conventional LLC half-bridge power converter includes a controller 50 that generates a predetermined frequency signal to control output of DC power, a power output unit 10 that receives the frequency signal of the controller 50 through a primary coil of a first transformer T1 and switches a DC voltage through two switching transistors FET1 and FET2 controlled by outputs of two secondary coils of the first transformer T1 to generate AC pulse signals, an LLC resonator 20 that resonates the AC pulse signals of the power output unit 10 through an inductor L1, a primary coil of a second transformer T2 and a resonant condenser C1, a rectifier 30 that rectifies the voltage across both terminal of a secondary coil of the second transformer T2 according to diodes D4 and D5 and smoothens the rectified voltage through a smoothing condenser C4 to output a DC voltage (+12V) for driving a load, and an output level feedback unit 40 that divides the output of the rectifier through voltage-dividing resistors R4 and R5, detects the output level of the rectifier 30 according to a level detecting element TL431 and feeds back the output of the level detecting element TL431 to the controller 50 through an opto-coupler PC1.
In the conventional LLC half-bridge power converter constructed as above, when the primary coil of the first transformer T1 is controlled according to frequency control of the controller 50, voltages are respectively induced to the two secondary coils of the first transformer T1 according to a DC voltage applied to the power converter and the two switching transistors FET1 and FET2 respectively generate pulse signals. Then, the LLC resonator 20 resonates the pulse signals and the rectifier 30 rectifies the resonated signal to output a DC voltage for driving a load. The operation of the LLC half-bridge power converter is well-known in the art so that detailed explanation thereof is omitted.
In the above-described conventional LLC half-bridge power converter, when the output level is detected by the voltage-dividing resistors R4 and R5 of the feedback unit 40 and the divided voltages exceed a set voltage of the level detecting element TL431, the level detecting element TL431 is turned on to operate the opto-coupler PC1. Upon the operation of the opto-coupler PC1, the controller 50 fixes a resonant frequency to fix the output level.
However, if the feedback unit 40 does not apply a feedback signal to the controller 50 due to circuit failure even though the output level exceeds the set output level DC +12V, the controller 50 cannot control the output level to be increased to result in generation of a secondary trouble.
The control method used in the aforementioned conventional LLC half-bridge power converter converts the power state of the power converter into a maximum power state at a time set by a soft-start function of the controller 50. Here, the output of the rectifier 30 is monitored by the feedback unit 40 and fed back to the controller 50 according to voltage and current set values and the controller 50 controls a set voltage output to be maintained. This control method causes a problem that the output voltage exceeds a set level if the feedback unit 40 does not operate because the output voltage detecting function is impaired or other problems are generated. This may damage a device using the power converter.
Furthermore, the controller that determines the performance of the DC power converter according to a method improved more than conventional flyback, forward and half-bridge methods has a low frequency in the range of 50 KHz to 350 KHz and is expensive. Accordingly, a small-volume power converter is difficult to obtain at low cost.